1. Field of the Invention
This invention relates to a method and apparatus for servicing a heat exchanger and, more particularly, to a method and apparatus for remotely mapping a tube sheet of such a heat exchanger.
2. Description of the Prior Art
In a pressurized water nuclear-powered electric generating system the heat generated by the nuclear reaction is transferred by a primary coolant that circulates through the reactor core to a steam generator where it is utilized to generate steam. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. A transverse plate called a tube sheet, located at the lower end of the cylindrical section, divides the steam generator into a primary side, which is the lower hemispherical section below the tube sheet, and a secondary side above the tube sheet. A vertical wall bisects the primary side into an inlet section and an outlet section. The tube sheet is a thick carbon steel plate with an array of hundreds of holes into which are inserted the ends of U-shaped tubes. One end of each U-shaped tube is inserted into a hole in the tube sheet which communicates with the inlet section of the primary side, and the other is inserted into a corresponding hole which communicates with the outlet section. The primary coolant is introduced under pressure into the inlet section of the primary side, circulates through the U-shaped tubes and exits through the outlet section. Water introduced into the secondary side of the steam generator circulates around the U-shaped tubes and is transformed into steam by the heat emanating from the primary coolant.
Occasionally, during operation of the steam generator leaks develop in some of the tubes. This is undesirable because the primary coolant can contain some radioactive contamination and any cross-feed of the reactor coolant into the secondary side of the generator, therefore, can contaminate the steam. It is not practical, however, to replace leaky tubing as it occurs, but instead the effected tubes are usually plugged at both ends. In view of the hundreds of tubes in a steam generator, plugging of a few does not appreciably effect the efficiency of heat transfer.
Eventually, however, in some cases, a sufficient number of tubes can become plugged to adversely effect heat transfer. In such cases, it may be desirable to either replace or retube the steam generator. In the retubing process, all the tube holes, including any plugged holes, are drilled out and spot-faced from the primary side, and the tubes are then pulled out from the secondary side. New tubes are inserted from the secondary side with tubing guides inserted in the tube ends to ease their passages through holes in transfer support plates and the tube sheet. The tube guides are then removed from the primary side and the ends of the tubes are aligned with the spot-faced end of the hole in the tube sheet, tack-rolled and then welded in place.
While space to maneuver is not a particular problem on the secondary side of the steam generator, the radius of the partitioned, hemispherical primary side is typically approximately 5 feet, which does not provide much working room, especially near the circumference of the tube sheet. In addition, the primary side is usually radioactive, which requires worker protection and limitation of exposure.
In order to minimize worker exposure, a computerized, automated manipulator has been designed to carry out most of the tooling operations required on the primary side. As described in patent application Ser. No. 952,431, filed Oct. 18, 1978, the automated manipulator is capable of positioning the required tooling at almost any location on the primary side of the tube sheet to effect the desired machining processes. However, experience has shown that the location of the tube sheet holes will vary from generator to generator. Accordingly, it is desirable to map the holes in each tube sheet prior to performing the tooling operations required in retubing a steam generator so that the remote manipulator can properly position its end-affecters to carry out the necessary operations.
Therefore, a mapping end-effector is desired that can precisely locate the center line of each hole in the primary side of the tube sheet. Furthermore, it is desirable that such a probe identify whether or not each hole mapped is plugged and the nature of the plugs. Additionally, it is desirable to have such a tool that can indicate the diameter of each of the holes to be mapped.